batman-adv: fix batman-adv header overhead calculation

batman-adv: fix batman-adv header overhead calculation

Batman-adv prepends a full ethernet header in addition to its own
header. This has to be reflected in the MTU calculation, especially
since the value is used to set dev->hard_header_len.

Introduced by 411d6ed93a5d0601980d3e5ce75de07c98e3a7de
("batman-adv: consider network coding overhead when calculating required mtu")

Reported-by: cmsv <[email protected]>
Reported-by: Martin Hundebøll <[email protected]>
Signed-off-by: Marek Lindner <[email protected]>
Signed-off-by: Antonio Quartulli <[email protected]>